Processes for producing acetic acid

ABSTRACT

Processes for the producing acetic acid and, in particular, to improved processes for removing a cation, such as lithium, and iodides from a low energy carbonylation process to produce purified acetic acid. In one embodiment, the cation, e.g., lithium, may be removed using a cationic exchanger prior to removing iodides using a metal-exchanged ion exchange resin. The present invention is also suited for removing at least one cation selected from the group consisting of Groups IA and IIA of the periodic table, quaternary nitrogen cations, and phosphorous-containing cations.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application is a continuation-in-part and claims thepriority of U.S. patent application Ser. No. 14/694,913, entitled“Processes For Producing Acetic Acid,” filed Apr. 23, 2015, and claimspriority from U.S. Provisional Patent Application Ser. No. 62/141,490,entitled “Processes For Producing Acetic Acid,” filed Apr. 1, 2015, thedisclosure of which are incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

This invention relates to processes for producing acetic acid and, inparticular, to improved processes for removing lithium cations, whichmay be derived from and/or generated by lithium compounds in thereaction medium, prior to removing iodides from a low energycarbonylation process to produce purified acetic acid.

BACKGROUND OF THE INVENTION

Among currently employed processes for synthesizing acetic acid, one ofthe most useful commercially is the catalyzed carbonylation of methanolwith carbon monoxide as taught in U.S. Pat. No. 3,769,329, incorporatedherein by reference in its entirety. The carbonylation catalyst containsrhodium, either dissolved or otherwise dispersed in a liquid reactionmedium or supported on an inert solid, along with a halogen-containingcatalyst promoter as exemplified by methyl iodide. The rhodium can beintroduced into the reaction system in any of many forms. Likewise,because the nature of the halide promoter is not generally critical, alarge number of suitable promoters, most of which are organic iodides,may be used. Most typically and usefully, the reaction is conducted bycontinuously bubbling carbon monoxide gas through a liquid reactionmedium in which the catalyst is dissolved.

A widely used and successful commercial process for synthesizing aceticacid involves the catalyzed carbonylation of methanol with carbonmonoxide. The catalyst contains rhodium and/or iridium and a halogenpromoter, typically methyl iodide. The reaction is conducted bycontinuously bubbling carbon monoxide through a liquid reaction mediumin which the catalyst is dissolved. The reaction medium comprises aceticacid, methyl acetate, water, methyl iodide and the catalyst. Commercialprocesses for the carbonylation of methanol include those described inU.S. Pat. No. 3,769,329, the entireties of which is incorporated hereinby reference. Another conventional methanol carbonylation processincludes the Cativa™ process, which is discussed in Jones, J. H. (2002),“The Cativa™ Process for the Manufacture of Acetic Acid,” PlatinumMetals Review, 44 (3): 94-105, the entirety of which is incorporatedherein by reference.

The AO™ process for the carbonylation of an alcohol to produce thecarboxylic acid having one carbon atom more than the alcohol in thepresence of a rhodium catalyst is disclosed in U.S. Pat. Nos. 5,001,259;5,026,908; and 5,144,068; and EP0161874, the entireties of which areincorporated herein by reference. As disclosed therein, acetic acid isproduced from methanol in a reaction medium containing methyl acetate(MeAc), methyl halide, especially methyl iodide (MeI), and rhodiumpresent in a catalytically effective concentration. These patentsdisclose that catalyst stability and the productivity of thecarbonylation reactor can be maintained at high levels, even at very lowwater concentrations, i.e., 4 weight percent or less, (despite the priorpractice of maintaining approximately 14-15 wt. % water) by maintainingin the reaction medium, along with a catalytically effective amount ofrhodium, at least a finite concentration of water, e.g., 0.1 wt. %, anda specified concentration of iodide ions over and above the iodide ionthat is present as hydrogen iodide. This iodide ion is a simple salt,with lithium iodide being preferred. The salt may be formed in situ, forexample, by adding lithium acetate, lithium carbonate, lithium hydroxideor other lithium salts of anions compatible with the reaction medium.The patents teach that the concentration of methyl acetate and iodidesalts are significant parameters in affecting the rate of carbonylationof methanol to produce acetic acid, especially at low reactor waterconcentrations. By using relatively high concentrations of the methylacetate and iodide salt, a high degree of catalyst stability and reactorproductivity is achieved even when the liquid reaction medium containswater in finite concentrations as low as 0.1 wt. %. Furthermore, thereaction medium employed improves the stability of the rhodium catalyst,i.e., resistance to catalyst precipitation, especially during theproduct recovery steps of the process. In these steps, distillation forthe purpose of recovering the acetic acid product tends to remove fromthe catalyst the carbon monoxide, which in the environment maintained inthe reaction vessel, is a ligand with stabilizing effect on the rhodium.

U.S. Pat. No. 5,144,068, the entirety of which is incorporated herein byreference, discloses a process for producing acetic acid by reactingmethanol with carbon monoxide in a liquid reaction medium containing arhodium (Rh) catalyst and comprising water, acetic acid, methyl iodide,and methyl acetate, wherein catalyst stability is maintained in thereaction by maintaining in said reaction medium during the course ofsaid reaction 0.1 wt. % to 14 wt. % of water together with (a) aneffective amount in the range of 2 wt. % to 20 wt. % of a catalyststabilizer selected from the group consisting of iodide salts which aresoluble in said reaction medium in effective concentration at reactiontemperature, (b) 5 wt. % to 20 wt. % of methyl iodide, and (c) 0.5 wt. %to 30 wt. % of methyl acetate. Suitable iodide salts may be a quaternaryiodide salt or an iodide salt of a member of the group consisting of themetals of Group IA and Group IIA of the Periodic Table.

Carbonyl impurities, such as acetaldehyde, that are formed during thecarbonylation of methanol may react with iodide catalyst promoters toform multi-carbon alkyl iodides, e.g., ethyl iodide, propyl iodide,butyl iodide, pentyl iodide, hexyl iodide, and the like. It is desirableto remove multi-carbon alkyl iodides from the reaction product becauseeven small amounts of these impurities in the acetic acid product tendto poison the catalyst used in the production of vinyl acetate, aproduct commonly produced from acetic acid.

Conventional techniques to remove such impurities include treating thecrude acid product streams with oxidizers, ozone, water, methanol,activated-carbon, amines, and the like. Such treatments may or may notbe combined with distillation of the acetic acid. The most typicalpurification treatment involves a series of distillations to yield asuitable purified acetic acid as the final product. It is also known toremove carbonyl impurities from organic streams by treating the organicstreams with an amine compound such as hydroxylamine, which reacts withthe carbonyl compounds to form oximes, followed by distillation toseparate the purified organic product from the oxime reaction products.However, the additional treatment of the purified acetic acid adds costto the process, and distillation of the treated acetic acid product canresult in additional impurities being formed.

While it is possible to obtain acetic acid of relatively high purity,the acetic acid product formed by the low-water carbonylation processand purification treatment described above frequently remains somewhatdeficient with respect to the permanganate time due to the presence ofsmall proportions of residual impurities. Because a sufficientpermanganate time is an important commercial test, which the acidproduct may be required to meet to be suitable for many uses, thepresence of impurities that decrease permanganate time is objectionable.Moreover, it has not been economically or commercially feasible toremove minute quantities of these impurities from the acetic acid bydistillation because some of the impurities have boiling points close tothat of the acetic acid product or halogen-containing catalystpromoters, such as methyl iodide. It has thus become important toidentify economically viable methods of removing impurities elsewhere inthe carbonylation process without contaminating the purified acetic acidor adding unnecessary costs.

Macroreticulated or macroporous strong acid cationic exchange resincompositions are conventionally utilized to reduce iodide contamination.Suitable exchange resin compositions, e.g., the individual beadsthereof, comprise both sites that are functionalized with a metal, e.g.,silver, mercury or palladium, and sites that remain in the acid form.Exchange resin compositions that have little or no metal-functionalitydo not efficiently remove iodides and, as such, are not conventionallyused to do so. Typically, metal-functionalized exchange resins areprovided in a fixed bed and a stream comprising the crude acetic acidproduct is passed through the fixed resin bed. In the metalfunctionalized resin bed, the iodide contaminants contained in the crudeacetic acid product are removed from the crude acid product stream.

U.S. Pat. No. 6,657,078 describes a low-water process that uses ametal-functionalized exchange resin to remove iodides. The referencealso avoids the use of a heavy ends column, resulting in an energysavings.

The metal-functionalization of exchange resin compositions ofteninvolves significant processing and expense, often costing orders ofmagnitude more than resins that are not metal-functionalized. Often theprocess steps associated with the functionalization varies very littlewith regard to the actual amount of metal that is deposited on theexchange resin. For example, the processing necessary to functionalize50% of the active sites of a quantity of exchange resin is quite similarto the processing necessary to functionalize 10% of the active sites ofthe same quantity of exchange resin. Because the entire quantity ofexchange resin requires processing, however, both the 50%-functionalizedexchange resin and the 10%-functionalized resin require significantlymore processing than the same quantity of non-functionalized resin.

Other ion exchange resins have been used to remove iodide impuritiesfrom acetic acid and/or acetic anhydride. There is disclosed in U.S.Pat. No. 5,220,058 the use of ion exchange resins having metal exchangedthiol functional groups for removing iodide impurities from acetic acidand/or acetic anhydride. Typically, the thiol functionality of the ionexchange resin has been exchanged with silver, palladium, or mercury.

In addition to iodide contaminants, metals from the walls of the vesselsused in the acetic acid production system often corrode and dissolveinto the crude acetic acid product compositions. Thus, conventionalcrude acid product streams often comprise corrosion metal contaminantsas well as iodide contaminants. These corrosion metals are known tointerfere with the carbonylation reaction or accelerate competingreactions such as the water-gas shift reaction. Typically, thesecorrosion metals may be removed from the process streams by passing thestreams through resin beds comprising standard macroreticular ormacroporous cationic exchange resins.

In a case where a silver, mercury or palladium exchanged resin isutilized, however, the soluble corrosion metal cations may detrimentallydisplace the metal-functionalized sites of the exchange resins. As such,these exchange sites are unable to capture/remove the iodidecontaminants. The lifetime of the functionalized resin, with regard toiodide removal, is shortened by the presence of corrosion metals. Oftena pre-determined portion of the sites of the exchange resin compositionare functionalized, thus leaving the remainder of the sites in the acidform. As a result, the acid sites capture much of the corrosion metalswhile many of the functionalized sites remain available for iodideremoval. Although this technique may improve the lifetime of exchangeresins, the partial functionalization of the pre-determined portion ofsites requires significant processing and resources.

In addition, it has been found that a problem associated with the use ofsilver-exchanged strong acid cation exchange resins is that the silvermay actually be displaced by corrosion metals, as described in U.S. Pat.No. 5,344,976. According to this patent, the metal ion contaminants inthe acid and/or anhydride may arise from corrosion or the use ofreagents in the up stream process. The patent describes the use of acationic exchanger in the acid form to remove at least a portion of themetal ion contaminants such as iron, potassium, calcium, magnesium, andsodium from a carboxylic acid stream prior to contacting the stream withthe exchanged strong acid cation exchange resin to remove C₁ to C₁₀alkyl iodide compounds, hydrogen iodide or iodide salts. However, thisprocess does not describe purification for low-energy and low-watercarbonylation processes as described above that may contain lithium andlarger alkyl iodide compounds, in addition to iodides.

In addition, other schemes introduce other contaminants that may need tobe removed from the product. For example, it has been well known in theart for some time that adding an alkali component such as KOH to thedrying column of a carbonylation purification process is useful toinhibit the buildup of HI in the column. See, e.g., US Pub. No.2013/0264186 and earlier references. However, this addition introduces apotassium cation into the process that can also displace the silver in asilver-exchanged strong acid cation exchange resin.

Other processes remove corrosion metal contaminants at different stagesof the process, for example from the reactant composition. U.S. Pat. No.4,894,477 describes a process that uses strongly acidic ion exchangeresins in the lithium form to remove corrosion metal contaminants. U.S.Pat. No. 5,731,252 describes contacting the catalyst solution with anion exchange resin bed, in the lithium form, while requiringsimultaneous addition of a sufficient amount of water to allow thecorrosion metal salts in the catalyst medium to dissociate so that ionexchange can occur and the corrosion metals can be removed from thereactor catalyst solution.

While the above-described processes have been successful, the needexists for process for improved processes for producing acetic acid, inparticular, low water and low energy processes and methods for removingcontaminants from those processes.

SUMMARY OF THE INVENTION

This invention relates to processes for the production of acetic acid.In one embodiment the present invention relates to process for producingacetic acid that involves carbonylating either methanol, dimethyl ether,methyl acetate, or some combination thereof in the presence of water inan amount from 0.1 to 14 wt. % based on the total weight, a rhodiumcatalyst, methyl iodide, and a lithium compound, to form a reactionmedium in a reactor; then separating the reaction medium to form aliquid recycle stream and a vapor product stream; separating the vaporproduct stream in up to 2 distillation columns in a primary purificationtrain to produce a crude acid product comprising acetic acid andlithium, which may be derived from and/or generated by the lithiumcompound in the reaction medium, then contacting the crude acetic acidproduct with a cationic exchanger in the acid form to produce anintermediate acid product; and finally contacting the intermediateacetic acid product with a metal-exchanged ion exchange resin havingacid cation exchange sites to produce a purified acetic acid. Theintermediate acetic acid product can comprise less lithium than thecrude acetic acid product, for example, the intermediate acetic acidproduct can comprise, in some embodiments, less than or equal to 100wppb lithium, based on the total weight of the intermediate acetic acidproduct. In some embodiments, the purified acetic acid product cancomprise less than or equal to 100 wppb lithium, and/or a metaldisplaced from the metal-exchanged ion exchange resin, e.g., silver,mercury, palladium and rhodium, in an amount of less than or equal to100 wppb, and/or less than or equal to 100 wppb iodides, based on thepurified total weight of the acetic acid product. In some embodiments,the process can further comprise a step of adding a potassium saltselected from the group consisting of potassium acetate, potassiumcarbonate, and potassium hydroxide to the distilled acetic acid productprior to distilling the distilled acetic acid product in a seconddistillation column, wherein at least a portion of the potassium isremoved by the cationic exchanger in the acid form.

In some embodiments, the process can further comprise condensing an lowboiling point overhead obtained from the first distillation column toform a heavy liquid phase and a light liquid phase, and wherein aportion of the heavy liquid phase is treated to remove at least onepermanganate reducing compound selected from the group consisting ofacetaldehyde, acetone, methyl ethyl ketone, butylaldehyde,crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde, and thealdol condensation products thereof.

In some embodiments, the step of separating the vapor product stream cancomprise distilling the vapor product stream in a first distillationcolumn to form a sidedraw comprising acetic acid; and then distillingthe sidedraw in a second distillation column to produce a crude acidproduct comprising acetic acid and cations selected from the groupconsisting of Groups IA and IIA of the periodic table, quaternarynitrogen cations, and phosphorous-containing cations, e.g., preferablylithium cations.

In some embodiments, the crude acetic acid product is removed as a sidestream from above the bottom of the second distillation column. This canbe, for example, a liquid stream. In other embodiments, the crude aceticacid product can be removed as a residue from the bottom of the seconddistillation column.

In other embodiments, the invention relates to a process for producingacetic acid comprising carbonylating at least one member selected fromthe group consisting of methanol, dimethyl ether, and methyl acetate inthe presence of water in an amount from 0.1 to 14 wt. %, a rhodiumcatalyst, methyl iodide and iodide salts, to form a reaction medium in areactor; separating the reaction medium to form a liquid recycle streamand a vapor product stream; separating the vapor product stream in up to2 distillation columns in a primary purification train to produce acrude acid product comprising acetic acid comprising at least one cationselected from the group consisting of Groups IA and IIA of the periodictable, quaternary nitrogen cations, and phosphorous-containing cations,wherein the at least one cation may be derived from and/or generated bya compound in the reaction medium, contacting the crude acetic acidproduct with a cationic exchanger in the acid form to produce anintermediate acid product; and contacting the intermediate acetic acidproduct with a metal-exchanged ion exchange resin having acid cationexchange sites to produce a purified acetic acid.

In still other embodiments, the invention relates to a process forremoving iodides from a liquid composition comprising a carboxylic acidor an anhydride thereof, greater than 10 wppb of C₁₀-C₁₄ alkyl iodides,iodide anions, and a cation selected from the group consisting of GroupIA and IIA metals and quaternary nitrogen cations, and quaternaryphosphorous-containing cations, which process comprises contacting saidliquid composition with a cationic exchanger in the acid form to producean intermediate product with a reduced concentration of cations selectedfrom the group consisting of Group IA and IIA metals, quaternarynitrogen cations, and phosphorous-containing cations; and contacting theintermediate product with a metal-exchanged ion exchange resin havingacid cation exchange sites comprising at least one metal selected fromthe group consisting of silver, mercury, palladium and rhodium toproduce a purified acetic acid product.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood in view of the appendednon-limiting figures, wherein:

FIG. 1 illustrates a process for producing acetic acid with a metalfunctionalized fixed resin bed for iodide removal.

FIG. 2 illustrates another process for producing acetic acid with ametal functionalized fixed resin bed for iodide removal.

DETAILED DESCRIPTION OF THE INVENTION

At the outset, it should be noted that in the development of any suchactual embodiment, numerous implementation—specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem related and business related constraints, which will vary fromone implementation to another. In addition, the processes disclosedherein can also comprise components other than those cited orspecifically referred to, as is apparent to one having average orreasonable skill in the art.

In the summary and this detailed description, each numerical valueshould be read once as modified by the term “about” (unless alreadyexpressly so modified), and then read again as not so modified unlessotherwise indicated in context. Also, in the summary and this detaileddescription, it should be understood that a concentration range listedor described as being useful, suitable, or the like, is intended thatany and every concentration within the range, including the end points,is to be considered as having been stated. For example, a range “from 1to 10” is to be read as indicating each and every possible number alongthe continuum between about 1 and about 10. Thus, even if specific datapoints within the range, or even no data points within the range, areexplicitly identified or refer to only a few specific data points, it isto be understood that inventors appreciate and understand that any andall data points within the range are to be considered to have beenspecified, and that inventors possessed knowledge of the entire rangeand all points within the range.

Throughout the entire specification, including the claims, the followingterms have the indicated meanings unless otherwise specified.

As used in the specification and claims, “near” is inclusive of “at.”The term “and/or” refers to both the inclusive “and” case and theexclusive “or” case, and is used herein for brevity. For example, amixture comprising acetic acid and/or methyl acetate may comprise aceticacid alone, methyl acetate alone, or both acetic acid and methylacetate.

All percentages are expressed as weight percent (wt. %), based on thetotal weight of the particular stream or composition present, unlessotherwise noted. Room temperature is 25° C. and atmospheric pressure is101.325 kPa unless otherwise noted.

For purposes herein: acetic acid may be abbreviated as “AcOH”;

-   -   acetaldehyde may be abbreviated as “AcH”;    -   methyl acetate may be abbreviated “MeAc”;    -   methanol may be abbreviated “MeOH”;    -   methyl iodide may be abbreviated as “MeI”;    -   hydrogen iodide may be abbreviated as “HI”;    -   carbon monoxide may be abbreviated “CO”; and    -   dimethyl ether may be abbreviated “DME”.

HI refers to either molecular hydrogen iodide or dissociated hydriodicacid when at least partially ionized in a polar medium, typically amedium comprising at least some water. Unless otherwise specified, thetwo are referred to interchangeably. Unless otherwise specified, HIconcentration is determined via acid-base titration using apotentiometric end point. In particular, HI concentration is determinedvia titration with a standard lithium acetate solution to apotentiometric end point. It is to be understood that for purposesherein, the concentration of HI is not determined by subtracting aconcentration of iodide assumed to be associated with a measurement ofcorrosion metals or other non H+ cations from the total ionic iodidepresent in a sample.

It is to be understood that HI concentration does not refer to iodideion concentration. HI concentration specifically refers to HIconcentration as determined via potentiometric titration.

This subtraction method is an unreliable and imprecise method todetermine relatively lower HI concentrations (i.e., less than about 5weight percent) due to the fact that it assumes all non-H+ cations (suchas cations of Fe, Ni, Cr, Mo) are associated with iodide anionexclusively. In reality, a significant portion of the metal cations inthis process can be associated with acetate anion. Additionally, many ofthese metal cations have multiple valence states, which adds even moreunreliability to the assumption on the amount of iodide anion whichcould be associated with these metals. Ultimately, this method givesrise to an unreliable determination of the actual HI concentration,especially in view of the ability to perform a simple titration directlyrepresentative of the HI concentration.

For purposes herein, an “overhead” or “distillate” of a distillationcolumn refers to at least one of the lower boiling condensable fractionswhich exits at or near the top, (e.g., proximate to the top), of thedistillation column, and/or the condensed form of that stream orcomposition. Obviously, all fractions are ultimately condensable, yetfor purposes herein, a condensable fraction is condensable under theconditions present in the process as readily understood by one of skillin the art. Examples of noncondensable fractions may include nitrogen,hydrogen, and the like. Likewise, an overhead stream may be taken justbelow the upper most exit of a distillation column, for example, whereinthe lowest boiling fraction is a non-condensable stream or represents ade-minimis stream, as would be readily understood by one of reasonableskill in the art.

The “bottoms” or “residuum” of a distillation column refers to one ormore of the highest boiling fractions which exit at or near the bottomof the distillation column, also referred to herein as flowing from thebottom sump of the column. It is to be understood that a residuum may betaken from just above the very bottom exit of a distillation column, forexample, wherein the very bottom fraction produced by the column is asalt, an unusable tar, a solid waste product, or a de-minimis stream aswould be readily understood by one of reasonable skill in the art.

For purposes herein, distillation columns comprise a distillation zoneand a bottom sump zone. The distillation zone includes everything abovethe bottom sump zone, i.e., between the bottom sump zone and the top ofthe column. For purposes herein, the bottom sump zone refers to thelower portion of the distillation column in which a liquid reservoir ofthe higher boiling components is present (e.g., the bottom of adistillation column) from which the bottom or residuum stream flows uponexiting the column. The bottom sump zone may include reboilers, controlequipment, and the like.

It is to be understood that the term “passages”, “flow paths”, “flowconduits”, and the like in relation to internal components of adistillation column are used interchangeably to refer to holes, tubes,channels, slits, drains, and the like, which are disposed through and/orwhich provide a path for liquid and/or vapor to move from one side ofthe internal component to the other side of the internal component.Examples of passages disposed through a structure such as a liquiddistributor of a distillation column include drain holes, drain tubes,drain slits, and the like, which allow a liquid to flow through thestructure from one side to another.

Average residence time is defined as the sum total of all liquid volumehold-up for a given phase within a distillation zone divided by theaverage flow rate of that phase through the distillation zone. Thehold-up volume for a given phase can include liquid volume contained inthe various internal components of the column including collectors,distributors and the like, as well as liquid contained on trays, withindowncomers, and/or within structured or random packed bed sections.

Lithium Removal

This invention relates to processes for the production of acetic acidand, in particular, to improved processes for removing cations, such aslithium, and iodides, including higher molecular weight iodides, forexample, C₁₀-C₁₄ alkyl iodides, from a low energy carbonylation process.The process is capable of removing at least one cation derived fromand/or generated by a compound in the reaction medium and these cationsmay be selected from the group consisting of Groups IA and IIA of theperiodic table, quaternary nitrogen cations, and phosphorous-containingcations. These cations may be derived from and/or generated by acompound in the reaction medium. In one embodiment, the process isdirected to removing a lithium cation derived from and/or generated bythe lithium compound in the reaction medium, such as lithium iodide orlithium acetate. According to the present invention the cation, andpreferably lithium cation, is removed prior to the iodide removal toprevent displacement in the metal ion-exchange resin.

With ever increasing cost pressures and higher energy prices, there hasbeen ever increasing motivation to simplify chemical manufacturingoperations and particularly to reduce the number of manufacturing steps.In this regard, it is noted that U.S. Pat. No. 5,416,237 discloses asingle zone distillation process for making acetic acid. Such processmodifications, while desirable in terms of energy costs, tend to placeincreasing demands on the purification train. In particular, fewerrecycles and distillations tend to introduce (or fail to remove) ahigher level of iodides and other promoters in the crude acid product,and particularly more iodides of a higher molecular weight. For example,octyl iodide, decyl iodide and dodecyl iodides may all be present in thecrude acid product as well as hexadecyl iodide; all of which aredifficult to remove by conventional techniques.

Low water and low energy processes for producing acetic acid by thecarbonylation of methanol have been developed which involve arhodium-catalyzed system operating at a water concentration of less thanor equal to 14 wt. %, and more preferably less than or equal to 4.1 wt.%, and utilizing up to 2 distillation columns in the primarypurification train. The primary purification train is directed atremoving bulk components, such as water, methyl acetate, methyl iodide,and hydrogen iodide, from the vapor product stream from thereactor/flash vessel to obtain acetic acid. This primary purificationtrain receives the majority of the vapor flow from the reactor andobtains acetic acid as a purified acetic acid. For example, the columnsof the primary purification train include the light ends column anddrying column. This primary purification train may exclude columns whosemain function is to remove minor components such as acetaldehyde,alkanes, and propionic acid.

The process for producing acetic acid in the reaction may use cations,such as a lithium cations, that have been found by the present inventorsto collect in the crude acid product. Unlike other metals that may bepresent in the crude acetic acid product that are the result ofcorrosion metals or metals from the up stream process, i.e. adding afterthe reactor, lithium cations may be derived from and/or generated by thelithium compound in the reaction medium. It was previously understoodthat the lithium compounds were less volatile and remained in the liquidrecycle from the flash vessel. It has now been discovered that lithiumcations derived from and/or generated by the lithium compound in thereaction medium may be entrained or be volatile enough to concentratewith the crude acetic acid product after purification in the primarypurification trains. These residual lithium cations may be difficult toremove and in the final metal-exchange guard bed may adversely replacemetals in the guard bed, resulting in poor iodide removal and increasedmetals displaced from the metal-exchanged ion exchange resin, e.g.silver, mercury, palladium and rhodium, in the purified acetic acid.Thus, the purified acetic acid may also have unacceptable levels ofiodides despite using a metal exchange guard bed. The present inventionprovides process for removing the cations, particularly the lithiumcations.

The source of the cation may be derived from or generated by a varietypromoters, co-catalysts, additives, in situ reactions, etc. For examplelow water and low energy processes that involve the use of a promotersuch as lithium iodide, which may form in situ following the addition oflithium acetate or other compatible lithium salts to the reactionmixture. Therefore, process streams may contain some quantity of lithiumions derived from and/or generated by the lithium compound in thereaction medium. It was previously understood that lithium ions did notaffect separation and purity in the primary purification train. Inaddition, since the process uses a maximum of 2 distillation columns inthe primary purification train and preferably the primary purificationdoes not include a column to remove heavy ends materials, the crude acidproduct may contain larger alkyl iodide compounds, e.g., C₁₀-C₁₄ alkyliodides, in addition to cations, such as lithium. In some embodiments,greater than or equal to 10% of the iodides present, or even greaterthan or equal to 50%, have an organic chain length of more than 10carbon atoms. Thus, the crude acid product may comprise C₁₀-C₁₄ alkyliodides in an amount of greater than or equal to 10 weight parts perbillion (wppb), e.g., greater than or equal to 20 wppb, greater than orequal to 50 wppb, greater than or equal to 100 wppb, greater than orequal to 1 wppm, or greater than or equal to 10 wppm. These higher alkyliodides may be in addition to the usual shorter chain length iodideimpurities found in the crude acid product of an iodide promotedcarbonylation process, including methyl iodide, HI, and hexyl iodide.Iodide impurities are typically removed from the crude acid productusing a metal-exchanged strong acid ion exchange resin in which themetal is silver or mercury, for example. However, it has been found thatthe silver or mercury in the metal-exchanged strong acid ion exchangeresin may be displaced by the residual lithium, resulting in lower resincapacity and efficiency and the potential for contaminating the productwith silver or mercury.

The cation in the crude acid product may result from the use of organicalkali salt ligands, such as organic lithium salt ligands, such as thosedescribed CN101053841 and CN1349855, the entire contents and disclosureof which are hereby incorporated by reference. CN101053841 describes aligand comprising lithium acetate or lithium oxalate. CN1349855describes a bimetallic catalyst having a metal lithium organic ligandcoordinating cis-dicarbonyl rhodium structure. The metal lithium organicligand may be a pyridine derivative, such as lithium pyridine-2-formate,lithium pyridine-3-formate, lithium pyridine-4-formate, lithiumpyridine-3-acetate, lithium pyridine-4-acetate, or lithiumpyridine-3-propionate. In fact, the lithium salt component of all ofthese ligands are believed to generate lithium iodide in situ within thereaction medium after exposure to methyl iodide at reaction temperaturesand pressures in the carbonylation reactor. At least some small portionof the lithium component may entrain into the purification system. Thus,the present invention may also remove lithium formed in situ from use ofthese types of organic ligands.

Cations may also be present as a result of the use of non-lithium salts,such as through the use of bimetallic Rh chelating catalysts that havean amine functionality, such as those described in CN1640543, the entirecontents and disclosure of which is hereby incorporated by reference.According to CN16040543 the cation species contains N and O donor atomsand is formed from aminobenzoic acid. The amine may quaternize withmethyl iodide in situ within the reaction medium at reaction temperatureand pressure to form a quaternary nitrogen cation. The quaternarynitrogen cation, similar to the lithium cation, may be carried throughwith the crude acid product and may be removed using the presentinvention prior to the metal-exchange guard beds.

The present invention therefore involves a low water and low energyprocess for producing acetic acid by the carbonylation of methanol,dimethyl ether, and/or methyl acetate in the presence of a waterconcentration from 0.1 to 14 wt. %, a metal catalyst, methyl iodide andlithium compounds, such as lithium acetate or lithium iodide. Theinvention utilizes up to 2 distillation columns in the primarypurification train and purifies the resulting acidic acid product with acationic exchanger in the acid form to remove residual lithium ionsderived from and/or generated by the lithium compound in the reactionmedium followed by treatment with a metal-exchanged ion exchange resinhaving acid cation exchange sites comprising at least one metal selectedfrom the group consisting of silver, mercury, palladium and rhodium. Themetal-exchanged ion exchange resin can have at least 1% of the strongacid exchange sites occupied by silver, mercury, palladium, and/orrhodium, e.g., at least 2% silver, mercury, palladium, and/or rhodium,at least 5% silver, mercury, palladium, and/or rhodium, at least 10%silver, mercury, palladium, and/or rhodium, or at least 20% silver,mercury, palladium, and/or rhodium. By using a cationic exchanger toremove lithium prior to the use of a resin having metal-exchanged strongacid cation sites, the displacement of silver, mercury, palladium and/orrhodium from the metal-exchanged sites by the lithium is reduced oreliminated for a process that utilizes up to 2 distillation columns inthe primary purification train.

Particularly preferred processes are those utilizing a cationicexchanger for removing lithium followed by a silver-exchanged cationicsubstrate for removing iodides. The crude acid product in many casesincludes organic iodides with a C₁₀ or more aliphatic chain length whichneed to be removed. Sometimes more than 10% of the iodides present,e.g., more than 15%, more than 30% or even more than 50%, have anorganic chain length of 10 carbon atoms or more.

Decyl iodides and dodecyl iodides are especially prevalent in theabsence of heavy ends and other finishing apparatus and are difficult toremove from the product. The silver-exchanged cationic substrates of thepresent invention typically remove over 90% of such iodides; oftentimesthe crude acid product has from 10 to 1000 wppb total iodide prior totreatment which would make the product unusable for iodide-sensitiveapplications.

An iodide level of 20 wppb to 1.5 wppm in the crude acid product priorto iodide removal treatment is typical; whereas the iodide removaltreatment is preferably operative to remove at least about 95% of thetotal iodide present. In a typical embodiment, lithium/iodide removaltreatment involves contacting the crude acid product with a cationicexchanger to remove 95 wt. % or more, e.g., 95 wt. % or more, 97 wt. %or more, 98 wt. % or more, 99 wt. % or more, or 99.5 wt. % or more ofthe lithium ions followed by contacting the crude acid product with asilver-exchanged sulfonic acid functionalized macroreticular ionexchange resin, wherein the product has an organic iodide content ofgreater than 100 wppb, e.g., greater than 100 wppb, greater than 200wppb, greater than 400 wppb, greater than 500 wppb, or greater than 1000wppb, prior to treatment and an organic iodide content of less than 10wppb, e.g., less than 10 wppb, less than 7 wppb, less than 5 wppb, lessthan 3 wppb, less than 2 wppb, less than 1 wppb, after contacting theresin.

Lithium has also been found to be entrained in the crude acid product inthe absence of heavy ends and other finishing apparatus. Even in verysmall amounts of 10 wppb of lithium in the crude acid product may causeproblem for removing iodides. The lithium in the acid-containing crudeacid product exiting the drying column of an acetic acid process, e.g.,the last column in the primary purification train, may be in an amountup to or equal to 10 wppm, e.g., up to or equal to 5 wppm, up to orequal to 1 wppm, up to or equal to 500 wppb, up to or equal to 300 wppb,or up to or equal to 100 wppb. In terms of ranges, the crude acidproduct may comprise lithium in an amount from 0.01 wppm to 10 wppm,e.g., from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm. By utilizinga cationic exchanger in the acid form before introducing the crude acidproduct to a metal-exchanged resin, significant amounts of lithium canbe removed. For example greater than or equal to 90 wt. % of the lithiumin the stream might be removed by the cationic exchanger, e.g., greaterthan 92, wt. %, greater than 95 wt. %, greater than 98 wt. %, or greaterthan 99 wt. %. Thus, the stream exiting the acid-form cationic exchangermay contain less than or equal to 50 wppb lithium, e.g., less than 25wppb lithium, less than or equal to 10 wppb, or less than or equal to 5wppb. Such removal of the lithium can greatly extend the life of themetal-exchanged resin.

Acetic Acid Production Systems

An exemplary acetic acid production process is described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The purification processes described herein may be useful incarbonylation processes that use methanol and/or methyl acetate (MeAc),methyl formate or dimethyl ether, or mixtures thereof, to produce aceticacid in the presence of a Group VIII metal catalyst, such as rhodium,and a halogen-containing catalyst promoter. A particularly usefulprocess is the low water rhodium-catalyzed carbonylation of methanol toacetic acid as exemplified in U.S. Pat. No. 5,001,259. Other metalcatalysts, e.g., iridium-based catalysts, are contemplated as well.

Generally, the metal component, e.g., rhodium component, of the catalystsystem is believed to be present in the form of a coordination compoundof rhodium with a halogen component providing at least one of theligands of such coordination compound. In addition to the coordinationof rhodium and halogen, it is also believed that carbon monoxidecoordinates with rhodium. The rhodium component of the catalyst systemmay be provided by introducing into the reaction zone rhodium in theform of rhodium metal, rhodium salts such as the oxides, acetates,iodides, carbonates, hydroxides, chlorides, etc., or other compoundsthat result in the formation of a coordination compound of rhodium inthe reaction environment.

The metal catalyst may comprise a Group VIII metal. Suitable Group VIIIcatalysts include rhodium and/or iridium catalysts. When a rhodiumcatalyst is used, the rhodium catalyst may be added in any suitable formsuch that rhodium is in the catalyst solution as an equilibrium mixtureincluding [Rh(CO)₂I₂]-anion, as is well known in the art. Iodide saltsoptionally maintained in the reaction mixtures of the processesdescribed herein may be in the form of a soluble salt of an alkali metalor alkaline earth metal, quaternary ammonium, phosphonium salt ormixtures thereof. In certain embodiments, the catalyst co-promoter islithium iodide, lithium acetate, or mixtures thereof. The saltco-promoter may be added as a non-iodide salt that generates an iodidesalt. The iodide catalyst stabilizer may be introduced directly into thereaction system. Alternatively, the iodide salt may be generated in-situsince under the operating conditions of the reaction system, a widerange of non-iodide salt precursors reacts with methyl iodide orhydroiodic acid in the reaction medium to generate the correspondingco-promoter iodide salt stabilizer. For additional detail regardingrhodium catalysis and iodide salt generation, see U.S. Pat. Nos.5,001,259; 5,026,908; 5,144,068 and 7,005,541, the entireties of whichare hereby incorporated by reference. The carbonylation of methanolutilizing iridium catalyst is well known and is generally described inU.S. Pat. Nos. 5,942,460, 5,932,764, 5,883,295, 5,877,348, 5,877,347 and5,696,284, the entireties of which are hereby incorporated by reference.

The halogen-containing catalyst promoter of the catalyst system consistsof a halogen compound comprising an organic halide. Thus, alkyl, aryl,and substituted alkyl or aryl halides can be used. Preferably, thehalogen-containing catalyst promoter is present in the form of an alkylhalide. Even more preferably, the halogen-containing catalyst promoteris present in the form of an alkyl halide in which the alkyl radicalcorresponds to the alkyl radical of the feed alcohol, which is beingcarbonylated. Thus, in the carbonylation of methanol to acetic acid, thehalide promoter may include methyl halide, and more preferably methyliodide.

The components of the reaction medium are maintained within definedlimits to ensure sufficient production of acetic acid. The reactionmedium contains a concentration of the metal catalyst, e.g., rhodiumcatalyst, in an amount from 200 to 3000 wppm, e.g., from 500 to 2000wppm, or from 600 to 1500 wppm. The concentration of water in thereaction medium is maintained under low water conditions, e.g., water inamount of less than or equal to 14 wt. %, from 0.1 wt. % to 14 wt. %,from 0.2 wt. % to 10 wt. % or most preferably from 0.25 wt. % to 5 wt.%. The concentration of methyl iodide in the reaction medium ismaintained to be from 1 to 25 wt. %, e.g., from 5 to 20 wt. %, from 4 to13.9 wt. %. The concentration of iodide salt, e.g., lithium iodide, inthe reaction medium is maintained to be from 1 to 25 wt. %, e.g., from 2to 20 wt. %, from 3 to 20 wt. %. The concentration of methyl acetate inthe reaction medium is maintained to be from 0.5 to 30 wt. %, e.g., from0.3 to 20 wt. %, from 0.6 to 4.1 wt. %. The following amounts are basedon the total weight of the reaction medium. The ranges disclosed in thisapplication include the endpoints, subranges and individual valuesunless otherwise stated.

The concentration of acetic acid in the reaction medium is generallygreater than or equal to 30 wt. %, e.g., greater than or equal to 40 wt.%, greater than or equal to 50 wt. %, or greater than or equal to 60 wt.%.

In embodiments, the process for producing acetic acid further includesintroducing a lithium compound into the reactor to maintain theconcentration of lithium acetate in an amount from 0.3 to 0.7 wt. % inthe reaction medium, wherein in an exemplary embodiment, in the reactionmedium the concentration of the rhodium catalyst is maintained in anamount from 200 to 3000 wppm in the reaction medium, the concentrationof water is maintained in amount from 0.1 to 4.1 wt. % in the reactionmedium, and the concentration of methyl acetate is maintained from 0.6to 4.1 wt. % in the reaction medium, based on the total weight of thereaction medium present within the carbonylation reactor.

In embodiments, the lithium compound introduced into the reactor isselected from the group consisting of lithium acetate, lithiumcarboxylates, lithium carbonates, lithium hydroxide, other organiclithium salts, and mixtures thereof. In embodiments, the lithiumcompound is soluble in the reaction medium. In an embodiment, lithiumacetate dihydrate may be used as the source of the lithium compound.

Lithium acetate reacts with hydrogen iodide according to the followingequilibrium reaction (I) to form lithium iodide and acetic acid:LiOAc+HI⇄LiI+HOAc  (I)

Lithium acetate is thought to provide improved control of hydrogeniodide concentration relative to other acetates, such as methyl acetate,present in the reaction medium. Without being bound by theory, lithiumacetate is a conjugate base of acetic acid and thus reactive towardhydrogen iodide via an acid-base reaction. This property is thought toresult in an equilibrium of the reaction (I) which favors reactionproducts over and above that produced by the corresponding equilibriumof methyl acetate and hydrogen iodide. This improved equilibrium isfavored by water concentrations of less than 4.1 wt. % in the reactionmedium. In addition, the relatively low volatility of lithium acetatecompared to methyl acetate allows the lithium acetate to remain in thereaction medium except for volatility losses and small amounts ofentrainment into the vapor crude product. In contrast, the relativelyhigh volatility of methyl acetate allows the material to distill intothe purification train, rendering methyl acetate more difficult tocontrol. Lithium acetate is much easier to maintain and control in theprocess at consistent low concentrations of hydrogen iodide.Accordingly, a relatively small amount of lithium acetate may beemployed relative to the amount of methyl acetate needed to controlhydrogen iodide concentrations in the reaction medium. It has furtherbeen discovered that lithium acetate is at least three times moreeffective than methyl acetate in promoting methyl iodide oxidativeaddition to the rhodium [I] complex. However, it has been discoveredthat lithium cations derived from and/or generated by the lithiumcompound in the reaction medium may be entrained or be volatile enoughto concentrate with the crude acetic acid product after purification inthe primary purification trains.

In embodiments, the concentration of lithium acetate in the reactionmedium is maintained at greater than or equal to 0.3 wt. %, or greaterthan or equal to 0.35 wt. %, or greater than or equal to 0.4 wt. %, orgreater than or equal to 0.45 wt. %, or greater than or equal to 0.5 wt.%, and/or in embodiments, the concentration of lithium acetate in thereaction medium is maintained at less than or equal to 0.7 wt. %, orless than or equal to 0.65 wt. %, or less than or equal to 0.6 wt. %, orless than or equal to 0.55 wt. %.

It has been discovered that an excess of lithium acetate in the reactionmedium can adversely affect the other compounds in the reaction medium,leading to decrease productivity. Conversely, it has been discoveredthat a lithium acetate concentration in the reaction medium below about0.3 wt. % is unable to maintain the desired hydrogen iodideconcentrations in the reaction medium of below 1.3 wt. %.

In embodiments, the lithium compound may be introduced continuously orintermittently into the reaction medium. In embodiments, the lithiumcompound is introduced during reactor start up. In embodiments, thelithium compound is introduced intermittently to replace entrainmentlosses.

Thus, in one embodiment there is provided a process for producing aceticacid comprising carbonylating at least one member selected from thegroup consisting of methanol, dimethyl ether, and methyl acetate in thepresence of water in an amount from 0.1 to 14 wt. %, a rhodium catalyst,methyl iodide and a lithium iodide, to form a reaction medium in areactor, wherein the concentration of lithium acetate in the reactionmedium is maintained at less than or equal to 0.7 wt. %, separating thereaction medium to form a liquid recycle stream and a vapor productstream, separating the vapor product stream in up to 2 distillationcolumns in a primary purification train to produce a crude acid productcomprising acetic acid and at least one lithium cation derived fromand/or generated by the lithium compound in the reaction medium,contacting the crude acetic acid product with a cationic exchanger inthe acid form to produce an intermediate acid product, and contactingthe intermediate acetic acid product with a metal-exchanged ion exchangeresin having acid cation exchange sites to produce a purified aceticacid.

In some embodiments, the desired reaction rates are obtained even at lowwater concentrations by maintaining in the reaction medium an ester ofthe desired carboxylic acid and an alcohol, desirably the alcohol usedin the carbonylation, and an additional iodide ion that is over andabove the iodide ion that is present as hydrogen iodide. A desired esteris methyl acetate. The additional iodide ion is desirably an iodidesalt, with lithium iodide (LiI) being preferred. It has been found, asdescribed in U.S. Pat. No. 5,001,259, that under low waterconcentrations, methyl acetate and lithium iodide act as rate promoters.

The carbonylation reaction of methanol to acetic acid product may becarried out by contacting the methanol feed with gaseous carbon monoxidebubbled through an acetic acid solvent reaction medium containing therhodium catalyst, methyl iodide promoter, methyl acetate, and additionalsoluble iodide salt, at conditions of temperature and pressure suitableto form the carbonylation product. It will be generally recognized thatit is the concentration of iodide ion in the catalyst system that isimportant and not the cation associated with the iodide, and that at agiven molar concentration of iodide the nature of the cation is not assignificant as the effect of the iodide concentration. Any metal iodidesalt, or any iodide salt of any organic cation, or other cations such asthose based on amine or phosphine compounds (optionally, ternary orquaternary cations), can be maintained in the reaction medium providedthat the salt is sufficiently soluble in the reaction medium to providethe desired level of the iodide. When the iodide is a metal salt,preferably it is an iodide salt of a member of the group consisting ofthe metals of Group IA and Group IIA of the periodic table as set forthin the “Handbook of Chemistry and Physics” published by CRC Press,Cleveland, Ohio, 2002-03 (83rd edition). In particular, alkali metaliodides are useful, with lithium iodide being particularly suitable. Inthe low water carbonylation process, the additional iodide ion over andabove the iodide ion present as hydrogen iodide is generally present inthe catalyst solution in amounts such that the total iodide ionconcentration is from 1 to 25 wt. % and the methyl acetate is generallypresent in amounts from 0.5 to 30 wt. %, and the methyl iodide isgenerally present in amounts from 1 to 25 wt. %. The rhodium catalyst isgenerally present in amounts from 200 to 3000 wppm.

The reaction medium may also contain impurities that should becontrolled to avoid byproduct formation. One impurity in the reactionmedium may be ethyl iodide, which is difficult to separate from aceticacid. Applicant has further discovered that the formation of ethyliodide may be affected by numerous variables, including theconcentration of acetaldehyde, ethyl acetate, methyl acetate and methyliodide in the reaction medium. Additionally, ethanol content in themethanol source, hydrogen partial pressure and hydrogen content in thecarbon monoxide source have been discovered to affect ethyl iodideconcentration in the reaction medium and, consequently, propionic acidconcentration in the final acetic acid product.

In embodiments, the propionic acid concentration in the acetic acidproduct may further be maintained below 250 wppm by maintaining theethyl iodide concentration in the reaction medium at less than or equalto 750 wppm without removing propionic acid from the acetic acidproduct.

In embodiments, the ethyl iodide concentration in the reaction mediumand propionic acid in the acetic acid product may be present in a weightratio from 3:1 to 1:2. In embodiments, the acetaldehyde:ethyl iodideconcentration in the reaction medium is maintained at a weight ratiofrom 2:1 to 20:1.

In embodiments, the ethyl iodide concentration in the reaction mediummay be maintained by controlling at least one of the hydrogen partialpressure, the methyl acetate concentration, the methyl iodideconcentration, and/or the acetaldehyde concentration in the reactionmedium.

In embodiments, the concentration of ethyl iodide in the reaction mediumis maintained/controlled to be less than or equal to 750 wppm, or e.g.,less than or equal to 650 wppm, or less than or equal to 550 wppm, orless than or equal to 450 wppm, or less than or equal to 350 wppm. Inembodiments, the concentration of ethyl iodide in the reaction medium ismaintained/controlled at greater than or equal to 1 wppm, or e.g., 5wppm, or 10 wppm, or 20 wppm, or 25 wppm, and less than or equal to 650wppm, or e.g., 550 wppm, or 450 wppm, or 350 wppm.

In embodiments, the weight ratio of ethyl iodide in the reaction mediumto propionic acid in the acetic acid product may range from 3:1 to 1:2,or e.g., from 5:2 to 1:2, or from 2:1 to 1:2, or from 3:2 to 1:2.

In embodiments, the weight ratio of acetaldehyde to ethyl iodide in thereaction medium may range from 20:1 to 2:1, or e.g., from 15:1 to 2:1,from 9:1 to 2:1, or from 6:1.

Typical reaction temperatures for carbonylation may be from 150 to 250°C., e.g., 160 to 240° C., 170-230° C. with the temperature range of 180to 225° C. being a preferred range. The carbon monoxide partial pressurein the reactor can vary widely but is typically from 2 to 30 atm, e.g.,from 3 to 10 atm. The hydrogen partial pressure in the reactor istypically from 0.05 to 2 atm, e.g., from 1 to 1.9 atm. In someembodiments, the present invention may be operated with a hydrogenpartial pressure from 0.3 to 2 atm, e.g., from 0.3 to 1.5 atm, or from0.4 to 1.5 atm. Because of the partial pressure of by-products and thevapor pressure of the contained liquids, the total reactor pressure mayrange from 15 to 40 atm. The production rate of acetic acid may be from5 to 50 mol/L·h, e.g., from 10 to 40 mol/L·h, and preferably 15 to 35mol/L·h.

Exemplary reaction and acetic acid recovery is shown in FIG. 1. Asshown, methanol-containing feed stream 101 and carbonmonoxide-containing feed stream 102 are directed to liquid phasecarbonylation reactor 104, in which the carbonylation reaction occurs toform acetic acid.

Carbonylation reactor 104 is preferably either a stirred vessel orbubble-column type vessel, with or without an agitator, within which thereacting liquid or slurry contents are maintained, preferablyautomatically, a predetermined level, which preferably remainssubstantially constant during normal operation. Into carbonylationreactor 104, fresh methanol, carbon monoxide, and sufficient water arecontinuously introduced as needed to maintain suitable concentrations inthe reaction medium.

In a typical carbonylation process, carbon monoxide is continuouslyintroduced into the carbonylation reactor, desirably below the agitator,which may be used to stir the contents. The gaseous feed preferably isthoroughly dispersed through the reacting liquid by this stirring means.Gaseous purge stream 106 desirably is vented from the reactor 104 toprevent buildup of gaseous by-products and to maintain a set carbonmonoxide partial pressure at a given total reactor pressure. In oneembodiment, the gaseous purge stream 106 contains low amounts ofhydrogen iodide of less than or equal to 1 wt. %, e.g., less than orequal to 0.9 wt. %, less than or equal to 0.8 wt. %, less than or equalto 0.7 wt. %, less than or equal to 0.5 wt. %, less than or equal to 0.3wt. %. Hydrogen iodide in excess of these amounts may increase the dutyon the scrubber to prevent hydrogen iodide from being purged. Thetemperature of the reactor may be controlled and the carbon monoxidefeed is introduced at a rate sufficient to maintain the desired totalreactor pressure. Stream 113 comprising the liquid reaction medium exitsreactor 104.

The acetic acid production system preferably includes primarypurification train 108 employed to recover the acetic acid and recyclecatalyst solution, methyl iodide, methyl acetate, and other systemcomponents within the process. Primary purification train 108 includelight ends column 124 and drying column 130, and the associated pumps,overhead receivers, condensers, etc. Thus, a recycled catalyst solution,such as stream 110 from flash vessel 112, and optionally one or more ofrecycle streams 114, 116, 118, and 120, also are introduced into thereactor 104. Of course, one or more of the recycle streams may becombined prior to being introduced into the reactor. The separationsystem also preferably controls water and acetic acid content in thecarbonylation reactor, as well as throughout the system, and facilitatesPRC removal.

Flash Vessel

The reaction medium is drawn off from the carbonylation reactor 104 at arate sufficient to maintain a constant level therein and is provided toflash vessel 112 via stream 113. In flash vessel 112, the crude productis separated in a flash separation step to obtain a vapor product stream122 comprising acetic acid and less volatile stream 110, e.g., a liquidrecycle stream, comprising a catalyst-containing solution (predominantlyacetic acid containing the rhodium and the iodide salt along with lesserquantities of methyl acetate, methyl iodide, and water), whichpreferably is recycled to the reactor, as discussed above. The vaporproduct stream 122 also comprises methyl iodide, methyl acetate, water,and permanganate reducing compounds (PRC's). Dissolved gases exiting thereactor and entering the flash vessel comprise a portion of the carbonmonoxide and may also contain gaseous by-products such as methane,hydrogen, and carbon dioxide. Such dissolved gases exit the flash vesselas part of the overhead stream.

In one embodiment, vapor product stream 122 comprises acetic acid,methyl iodide, methyl acetate, water, acetaldehyde, and hydrogen iodide.In one embodiment, vapor product stream 122 comprises acetic acid in anamount from 45 to 75 wt. %, methyl iodide in an amount from 20 to 50 wt.%, methyl acetate in an amount of less than or equal to 9 wt. %, andwater in an amount of less than or equal to 15 wt. %, based on the totalweight of the vapor product stream. In another embodiment, vapor productstream 122 comprises acetic acid in an amount from 45 to 75 wt. %,methyl iodide in an amount from 24 to less than 36 wt. %, methyl acetatein an amount of less than or equal to 9 wt. %, and water in an amount ofless than or equal to 15 wt. %, based on the total weight of the vaporproduct stream. More preferably, vapor product stream 122 comprisesacetic acid in an amount from 55 to 75 wt. %, methyl iodide in an amountfrom 24 to 35 wt. %, methyl acetate in an amount from 0.5 to 8 wt. %,and water in an amount from 0.5 to 14 wt. %. In yet a further preferredembodiment, vapor product stream 112 comprises acetic acid in an amountfrom 60 to 70 wt. %, methyl iodide in an amount from 25 to 35 wt. %,methyl acetate in an amount from 0.5 to 6.5 wt. %, and water in anamount from 1 to 8 wt. %. The acetaldehyde concentration in the vaporproduct stream may be in an amount from 0.005 to 1 wt. %, based on thetotal weight of the vapor product stream, e.g., from 0.01 to 0.8 wt. %,or from 0.01 to 0.7 wt. %. In some embodiments the acetaldehyde may bepresent in amounts less than or equal to 0.01 wt. %. Vapor productstream 122 may comprise hydrogen iodide in an amount less than or equalto 1 wt. %, based on the total weight of the vapor product stream, e.g.,less than or equal to 0.5 wt. %, or less than or equal to 0.1 wt. %.Vapor product stream 122 is preferably substantially free of, i.e.,contains less than or equal to 0.0001 wt. %, propionic acid, based onthe total weight of the vapor product stream.

Less volatile stream 110 comprises acetic acid, the metal catalyst,corrosion metals, as well as other various compounds. In one embodiment,liquid recycle stream comprises acetic acid in an amount from 60 to 90wt. %, metal catalyst in an amount from 0.01 to 0.5 wt. %; corrosionmetals (e.g., nickel, iron and chromium) in a total amount from 10 to2500 wppm; lithium iodide in an amount from 5 to 20 wt. %; methyl iodidein an amount from 0.5 to 5 wt. %; methyl acetate in an amount from 0.1to 5 wt. %; water in an amount from 0.1 to 8 wt. %; acetaldehyde in anamount of less than or equal to 1 wt. % (e.g., from 0.0001 to 1 wt. %acetaldehyde); and hydrogen iodide in an amount of less than or equal to0.5 wt. % (e.g., from 0.0001 to 0.5 wt. % hydrogen iodide).

Recovery of Acetic Acid

The distillation and recovery of acetic acid is not particularly limitedfor the purposes of the present invention. In one exemplary embodiment,there is provided a process for producing acetic acid comprisingseparating a reaction medium formed in a reactor in a flash vessel toform a liquid recycle and a vapor product stream, distilling the vaporproduct stream in a first column to obtain a side stream and a lowboiling overhead vapor stream comprising water in an amount of greaterthan or equal to 5 wt. %, condensing the low boiling overhead vaporstream and biphasically separating the condensed stream to form a heavyliquid phase and a light liquid phase, optionally treating a portion ofthe heavy liquid phase and/or the light liquid phase to remove at leastone PRC, distilling the side stream in a second column to obtain a crudeacetic acid product in the bottoms or a liquid side stream from thesecond removed within 5 trays from the bottom of the second column;contacting the crude acetic acid product with a cationic exchanger inthe acid form to produce an intermediate acid product; and contactingthe intermediate acetic acid product with a metal-exchanged ion exchangeresin having acid cation exchange sites to produce a purified aceticacid. Various embodiments of primary purification train having up to 2distillation columns is further described herein.

First Column

The overhead stream from flash vessel 112 is directed to the light endscolumn 124 as vapor product stream 122, where distillation yields alow-boiling overhead vapor stream 126, a sidedraw 128 that containsacetic acid, and a high boiling residue stream 116. In one embodiment,vapor product stream 122 may comprise acetic acid, methyl acetate,water, methyl iodide, and acetaldehyde, along with other impurities suchas hydrogen iodide and crotonaldehyde, and byproducts such as propionicacid. Acetic acid removed via sidedraw 128 preferably is subjected tofurther purification, such as in drying column 130 for selectiveseparation of acetic acid from water.

Light ends column 124 also preferably forms residuum or bottoms stream116, which comprises primarily acetic acid and water. Since light endsbottoms stream 116 typically comprises some residual catalyst, it may bebeneficial to recycle all or a portion of light ends bottoms stream 116to reactor 104. Optionally, light ends bottoms stream 116 may becombined with the catalyst phase 110 from flash vessel 112 and returnedtogether to reactor 104, as shown in FIG. 1. Although the concentrationof acetic acid may be relatively high in high boiling residue stream116, the mass flow of the high boiling residue stream 116 relative toside stream 128 is very small. In embodiments, the mass flow of theboiling residue stream 116 is less than or equal to 0.75% of side stream128, e.g., less than or equal to 0.55%, or less than or equal to 0.45%.

In one embodiment, low-boiling overhead vapor stream 126 comprises waterin amount greater than or equal to 5 wt. %, e.g., greater than or equalto 10 wt. %, or greater than or equal to 25 wt. %. The amount of watermay be up to 80 wt. %. In terms of ranges, water concentration in theoverhead may be from 5 wt. % to 80 wt. %, e.g., from 10 wt. % to 70 wt.% or from 25 wt. % to 60 wt. %. Reducing water concentration to lessthan 5 wt. % is not advantageous because this results in a large recycleof acetic acid back to the reaction system which then sets up a largerecycle through the entire purification system. In addition to water,low-boiling overhead vapor stream 126 may also comprise methyl acetate,methyl iodide, and carbonyl impurities, which are preferablyconcentrated in the overhead to be removed from acetic acid in sidestream 128. These carbonyl impurities may also be referred to herein asPRC's.

As shown, low-boiling overhead vapor stream 126 preferably is condensedand directed to an overhead phase separation unit, as shown by overheaddecanter 134. Conditions are desirably maintained such that thecondensed low-boiling overhead vapor stream 126, once in decanter 134,may separate to form a light liquid phase 138 and a heavy liquid phase118. The phase separation should be maintain two separate phase, withoutforming a third phase or emulsion between the phases. An offgascomponent may be vented via line 136 from decanter 134. In embodiments,the average residence time of the condensed low-boiling overhead vaporstream 126 in overhead decanter 134 is greater than or equal to 1minute, e.g., greater than or equal to 3 minutes, greater than or equalto 5 minutes, greater than or equal to 10 minutes, and/or the averageresidence time is less than or equal to 60 minutes, e.g., less than orequal to 45 minutes, or less than or equal to 30 minutes, or less thanor equal to 25 minutes.

Although the specific compositions of the light phase stream 138 mayvary widely, some preferred compositions are provided below in Table 1.

TABLE 1 Exemplary Light Liquid Phase from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) HOAc 1-40 1-25 5-15 Water 50-90  50-80 60-80  PRC's  <5 <3 <1 MeI <10 <5 <3 MeAc 1-50 1-25 1-15

In one embodiment, overhead decanter 134 is arranged and constructed tomaintain a low interface level to prevent an excess hold up of methyliodide. Although the specific compositions of heavy liquid phase 118 mayvary widely, some exemplary compositions are provided below in Table 2.

TABLE 2 Exemplary Heavy Liquid Phase from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) Water 0.01-2  0.05-1   0.1-0.9 MethylAcetate 0.1-25 0.5-20  0.7-15  Acetic Acid 0.1-10 0.2-8   0.5-6   PRC's<5 <3 <1 Methyl Iodide  40-98 50-95 60-85

The density of the heavy liquid phase 118 may be from 1.3 to 2, e.g.,from 1.5 to 1.8, from 1.5 to 1.75 or from 1.55 to 1.7. As described inU.S. Pat. No. 6,677,480, the measured density in the heavy liquid phase118 correlates with the methyl acetate concentration in the reactionmedium. As density decreases, the methyl acetate concentration in thereaction medium increases. In one embodiment of the present invention,heavy liquid phase 118 is recycled to the reactor and the light liquidphase 138 is controlled to be recycled through the same pump. It may bedesirable to recycle a portion of the light liquid phase 138 that doesnot disrupt the pump and maintains a density of the combined lightliquid phase 138 and heavy liquid phase of greater than or equal to 1.3,e.g., greater than or equal to 1.4, greater than or equal to 1.5, orgreater than or equal to 1.7. As described herein, a portion of theheavy liquid phase 118 may be treated to remove impurities such asacetaldehyde.

As shown in FIG. 1, the light phase exits decanter 134 via stream 138. Afirst portion, e.g., aliquot portion, of light phase stream 138 isrecycled to the top of the light ends column 124 as reflux stream 140.In other embodiments a portion of the heavy liquid phase 118 may also berefluxed (not shown) to the light ends column 124.

PRC Removal System

As described herein the light ends column 124 is part of the primarypurification train. In some embodiments, a portion of light liquid phaseand/or heavy liquid phase may be separated and directed to acetaldehydeor PRC removal system 132 to recover methyl iodide and methyl acetate,while removing acetaldehyde. For purposes of the present invention, theacetaldehyde or PRC removal system 132 is not part of the primarypurification train.

As shown in Tables 1 and 2, light liquid phase 133 and/or heavy liquidphase 118 each contain PRC's and the process may include removingcarbonyl impurities, such as acetaldehyde, that deteriorate the qualityof the acetic acid product and may be removed in suitable impurityremoval columns and absorbers as described in U.S. Pat. Nos. 6,143,930;6,339,171; 7,223,883; 7,223,886; 7,855,306; 7,884,237; 8,889,904; and USPub. Nos. 2006/0011462, which are incorporated herein by reference intheir entirety. Carbonyl impurities, such as acetaldehyde, may reactwith iodide catalyst promoters to form alkyl iodides, e.g., ethyliodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, etc.Also, because many impurities originate with acetaldehyde, it isdesirable to remove carbonyl impurities from the liquid light phase.

The portion of light liquid phase 138 and/or heavy liquid phase 118 fedto the acetaldehyde or PRC removal system 132 via lines 142 and 143,respectively, may vary from 1% to 99% of the mass flow of either thelight liquid phase 138 and/or heavy liquid phase 118, e.g., from 1 to50%, from 2 to 45%, from 5 to 40%, 5 to 30% or 5 to 20%. Also in someembodiments, a portion of both the light liquid phase 138 and heavyliquid phase 118 may be fed to the acetaldehyde or PRC removal system132. The portion of the light liquid phase 138 not fed to theacetaldehyde or PRC removal system 132 may be refluxed to the firstcolumn or recycled to the reactor, as described herein. The portion ofthe heavy liquid phase 118 not fed to the acetaldehyde or PRC removalsystem 132 may be recycled to the reactor. Although a portion of heavyliquid phase 118 may be refluxed to the light ends column, it is moredesirable to return the methyl iodide enriched heavy liquid phase 118 tothe reactor.

In one embodiment, a portion of light liquid phase 138 and/or heavyliquid phase 118 is fed to a distillation column which enriches theoverhead thereof to have acetaldehyde and methyl iodide. Depending onthe configuration, there may be two separate distillation columns, andthe overhead of the second column may be enriched in acetaldehyde andmethyl iodide. Dimethyl ether, which may be formed in-situ, may also bepresent in the overhead. The overhead may be subject to one or moreextraction stages to remove a raffinate enriched in methyl iodide and anextractant. A portion of the raffinate may be returned to thedistillation column, first column, overhead decanter and/or reactor. Forexample, when the heavy liquid phase 118 is treated in the PRC removalsystem 132, it may be desirable to return a portion the raffinate toeither the distillation column or reactor. Also, for example, when lightliquid phase 138 is treated in the PRC removal system 132, it may bedesirable to return a portion the raffinate to either the first column,overhead decanter, or reactor. In some embodiments, the extractant maybe further distilled to remove water, which is returned to the one ormore extraction stages. The column bottoms, which contains more methylacetate and methyl iodide than light liquid phase 138, may also berecycled to reactor 104 and/or refluxed to light ends column 124.

Drying Column

Returning to the primary purification train, in addition to the overheadphase, the light ends column 124 also forms an acetic acid sidedraw 128,which preferably comprises primarily acetic acid and water. In order tomaintain an efficient product separation, it is important that thecomposition of the sidedraw 128 does not vary or fluctuate significantlyduring normal operation. By does not vary or fluctuate significantly itis meant that the concentration of the one or more C₁-C₁₄ alkyl iodidesand the concentration of methyl acetate is ±0.9% of the waterconcentration in the side stream, e.g., ±0.7%, ±0.5%, ±0.4%, ±0.3%,±0.2%, or ±0.1%. The water concentration in the side stream may be from1 to 3 wt. %, e.g., preferably from 1.1 to 2.5 wt. %. For example, whenthe water concentration is 2.5 wt. %, the concentration of C₁-C₁₄ alkyliodides is from 1.6 to 3.4 wt. %, and the concentration of methylacetate is from 1.6 to 3.4 wt. %.

Optionally, a portion of the sidedraw 128 may be recirculated to thelight ends column, preferably to a point below where sidedraw 128 wasremoved from light ends column, in order to improve the separation (notshown).

Since sidedraw 128 contains water in addition to acetic acid, sidedraw128 from the light ends column 124 preferably is directed to dryingcolumn 130, in which the acetic acid and water are separated from oneanother. As shown, drying column 130, separates acetic acid sidedraw 128to form overhead stream 144 comprised primarily of water and bottomsstream 146 comprised primarily of acetic acid. Overhead stream 144preferably is cooled and condensed in a phase separation unit, e.g.,decanter 148, to form a light phase 150 and a heavy phase 122. As shown,a portion of the light phase is refluxed, as shown by stream 152 and theremainder of the light phase is returned to the reactor 104, as shown bystream 120. The heavy phase, which typically is an emulsion comprisingwater and methyl iodide, preferably is returned in its entirety to thereactor 104, as shown by stream 122, optionally after being combinedwith stream 120.

Exemplary compositions for the light phase of the drying column overheadare provided below in Table 3.

TABLE 3 Exemplary Light Phase Compositions from Drying Column Overheadconc. (Wt. %) conc. (Wt. %) conc. (Wt. %) HOAc 1-20 1-15 1-10 Water50-90  60-90  70-90  MeI <10 <5 <3 MeAc 1-20 1-15 1-10

In certain embodiments, as discussed, minor amounts of an alkalicomponent such as KOH can be added to sidedraw 128 via line 160 prior toentering the drying column 130. In other embodiments, the alkalicomponent might also be added to the drying column 130 at the sameheight level as the stream 128 entering the drying column 130 or at aheight above the height level height level as the stream 128 enteringthe drying column 130. Such addition can neutralize HI in the column.

Drying column bottoms stream 146 preferably comprises or consistsessentially of acetic acid. In preferred embodiments, drying columnbottoms stream 146 comprises acetic acid in an amount greater than 90wt. %, e.g., greater than 95 wt. % or greater than 98 wt. %. Inembodiments, this stream may also be essentially anhydrous, for example,containing less than 0.2 wt. % water, e.g., less than 0.15 wt. % water,less than 0.12 wt. % water, less than 0.1 wt. % water, or less than 0.05wt. % water. However, as discussed, the stream may contain varyinglevels of impurities.

In FIG. 1, the crude acid product is withdrawn as a residue in dryingcolumn bottoms stream 146. In embodiments, bottoms stream 146 compriseslithium in an amount of up to or equal to 10 wppm, e.g., up to or equalto 5 wppm, up to or equal to 1 wppm, up to or equal to 500 wppb, up toor equal to 300 wppb, or up to or equal to 100 wppb. In terms of ranges,the crude acid product may comprise lithium in an amount from 0.01 wppmto 10 wppm, e.g., from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm.In terms of ranges, the side stream 170 may comprise lithium in anamount from 0.01 wppm to 10 wppm, e.g., from 0.05 wppm to 5 wppm or from0.05 wppm to 1 wppm. In addition, bottoms stream 146 comprises water inan amount of less than or equal to 0.2 wt. %, e.g., less than or equalto 0.15 wt. %, or less than or equal to 0.12 wt. %, or less than orequal to 0.11 wt. %. In one embodiment, the crude acetic acid productmay be essentially anhydrous. In further embodiments, it is preferrednot to dilute the crude acid product with an aqueous diluent, such aswater.

Thus, in one embodiment there is provided a process for producing aceticacid comprising separating a reaction medium formed in a reactor in aflash vessel to form a liquid recycle and a vapor product stream,distilling the vapor product stream in a first column to obtain a sidestream and a low boiling overhead vapor stream comprising water in anamount of greater than or equal to 5 wt. %, condensing the low boilingoverhead vapor stream and biphasically separating the condensed streamto form a heavy liquid phase and a light liquid phase, optionallytreating a portion of the heavy liquid phase and/or the light liquidphase to remove at least one PRC, distilling the side stream in a secondcolumn to obtain a bottoms comprising acetic acid and in an amount of upto or equal to 10 wppm of lithium, contacting the crude acetic acidproduct with a cationic exchanger in the acid form to produce anintermediate acid product, and contacting the intermediate acetic acidproduct with a metal-exchanged ion exchange resin having acid cationexchange sites to produce a purified acetic acid.

As shown in FIG. 2, in certain embodiments, the crude acid product fromthe drying column 130 may be taken from a side stream 170 at a positionslightly above the bottom 172 of the column 130. In one embodiment, sidestream 170 is withdrawn within 5 trays from the bottom 172 of the column130, e.g., within 4 trays from the bottom of the column 130, within 3trays from the bottom of the column 130, or within 2 trays from thebottom of the column 130. In some embodiments, the side stream 170 iswithdrawn a position between 2 and 5 trays from the bottom 172 of thecolumn 130, e.g., a position between 3 and 5 trays from the bottom ofthe column 130, or position between 3 and 4 trays from the bottom of thecolumn 130. Side stream 170 may be withdrawn in the liquid phase so thatthe lithium cation would be similar to withdrawing the lithium cationconcentration in the drying columns bottoms stream 146. A liquid sidestream 170 may be advantageous over a vapor stream in terms of consumingless energy. When a liquid side stream 170 is used then other impuritiessuch as heavy carbonyl containing groups, i.e. propionic acid, mayadvantageously concentrate in the bottoms stream 174. Residue stream 174may be discarded or purged from the process 100.

Side stream 170 contains the crude acetic acid product that is sent tocationic exchange resin to remove lithium. In embodiments, side stream170 comprises lithium in an amount of up to or equal to 10 wppm, e.g.,up to or equal to 5 wppm, up to or equal to 1 wppm, up to or equal to500 wppb, up to or equal to 300 wppb, or up to or equal to 100 wppb. Interms of ranges, the crude acid product may comprise lithium in anamount from 0.01 wppm to 10 wppm, e.g., from 0.05 wppm to 5 wppm or from0.05 wppm to 1 wppm. In terms of ranges, the side stream 170 maycomprise lithium in an amount from 0.01 wppm to 10 wppm, e.g., from 0.01wppm to 8 wppm, from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm. Inaddition, side stream 170 comprises water in an amount of less than orequal to 0.2 wt. %, e.g., less than or equal to 0.15 wt. %, or less thanor equal to 0.12 wt. %, or less than or equal to 0.11 wt. %.

Therefore, in another embodiment, there is provided a process forproducing acetic acid comprising separating a reaction medium formed ina reactor in a flash vessel to form a liquid recycle and a vapor productstream, distilling the vapor product stream in a first column to obtaina first side stream and a low boiling overhead vapor stream comprisingwater in an amount of greater than or equal to 5 wt. %, condensing thelow boiling overhead vapor stream and biphasically separating thecondensed stream to form a heavy liquid phase and a light liquid phase,optionally treating a portion of the heavy liquid phase and/or the lightliquid phase to remove at least one PRC, distilling the first sidestream in a second column to obtain a second side stream comprisingacetic acid and lithium in an amount of up to or equal to 10 wppm,wherein the second side stream is a liquid, contacting the crude aceticacid product with a cationic exchanger in the acid form to produce anintermediate acid product, and contacting the intermediate acetic acidproduct with a metal-exchanged ion exchange resin having acid cationexchange sites to produce a purified acetic acid.

In the present invention, the crude acid product withdrawn from thebottoms or the side draw is further processed to remove lithium derivedfrom and/or generated by the lithium compound in the reaction medium, bypassing through cationic exchanger in the acid form and then throughmetal functionalized iodide removal ion exchange resins, prior to beingstored or transported for commercial use. As described herein, cationicexchanger in the acid form are suitable for removing components derivedfrom and/or generated by compounds in the reaction medium thatconcentrate in the crude acid product. Once these components, and inparticular lithium, are removed the iodides may be removed by metalfunctionalized iodide removal ion exchange resins.

Iodide Removal Beds/Use of Ion Exchange Resins

According to the present process, carboxylic acid streams, e.g., aceticacid streams, that are contaminated with halides (e.g., iodides) andlithium derived from and/or generated by compounds in the reactionmedium may be contacted with an acid-form cationic exchange resinfollowed by a metal-exchanged ion exchange resin having acid cationexchange sites comprising at least one metal selected from the groupconsisting of silver, mercury, palladium and rhodium under a range ofoperating conditions. Preferably, the ion exchange resin compositionsare provided in fixed beds. The use of fixed iodide removal beds topurify contaminated carboxylic acid streams is well documented in theart (see, for example, U.S. Pat. Nos. 4,615,806; 5,653,853; 5,731,252;and 6,225,498, which are hereby incorporated by reference in theirentireties). Generally, a contaminated liquid carboxylic acid stream iscontacted with the aforementioned ion exchange resin compositions, byflowing through a series of static fixed beds. The lithium contaminantsare removed by the cationic exchanger in the acid form. The halidecontaminants, e.g., iodide contaminants, are then removed by reactionwith the metal of the metal-exchanged ion exchange resin to form metaliodides. In some embodiments, hydrocarbon moieties, e.g., methyl groups,that may be associated with the iodide may esterify the carboxylic acid.For example, in the case of acetic acid contaminated with methyl iodide,methyl acetate would be produced as a byproduct of the iodide removal.The formation of this esterification product typically does not have adeleterious effect on the treated carboxylic acid stream.

Similar iodide contamination issues may exist in acetic anhydridemanufactured via a rhodium-iodide catalyst system. Thus, the inventiveprocess may alternatively be utilized in the purification of crudeacetic anhydride product streams.

Suitable acid-form cation exchangers for removing metal ion contaminantsin the present invention may comprise strong acid cation exchangeresins, for example strong acid macroreticular or macroporous resins,for example Amberlyst® 15 resin (DOW), Purolite C145, or Purolite CT145.The resin may also be an acid-form strong acid cation exchangemesoporous resin. Chelating resins and zeolites may also be used.

Suitably stable ion exchange resins utilized in connection with thepresent invention for preparing silver or mercury-exchanged resins foriodide removal typically are of the “RSO3H” type classified as “strongacid,” that is, sulfonic acid, cation exchange resins of themacroreticular (macroporous) type. Particularly suitable ion exchangesubstrates include Amberlyst® 15, Amberlyst® 35 and Amberlyst® 36 resins(DOW) suitable for use at elevated temperatures. Other stable ionexchange substrates such as zeolites may be employed, provided that thematerial is stable in the organic medium at the conditions of interest,that is, will not chemically decompose or release silver or mercury intothe organic medium in unacceptable amounts. Zeolite cationic exchangesubstrates are disclosed, for example, in U.S. Pat. No. 5,962,735, thedisclosure of which is incorporated herein by reference.

At temperatures greater than about 50° C., the silver or mercuryexchanged cationic substrate may tend to release small amounts of silveror mercury on the order of 500 wppb or less and thus the silver ormercury exchanged substrate is chemically stable under the conditions ofinterest. More preferably, silver losses are less than 100 wppb into theorganic medium and still more preferably less than 20 wppb into theorganic medium. Silver losses may be slightly higher upon start up. Inany event, if so desired a bed of acid form cationic material may beplaced downstream of the silver or mercury exchange material in additionto the bed of acid form cationic material upstream of the silver ormercury exchange material, to catch any silver or mercury released.

The pressures during the contacting steps with the exchange resins arelimited only by the physical strength of the resins. In one embodiment,the contacting is conducted at pressures ranging from 0.1 MPa to 1 MPa,e.g., from 0.1 MPa to 0.8 MPa or from 0.1 MPa to 0.5 MPa. Forconvenience, however, both pressure and temperature preferably may beestablished so that the contaminated carboxylic acid stream is processedas a liquid. Thus, for example, when operating at atmospheric pressure,which is generally preferred based on economic considerations, thetemperature may range from 17° C. (the freezing point of acetic acid) to118° C. (the boiling point of acetic acid). It is within the purview ofthose skilled in the art to determine analogous ranges for productstreams comprising other carboxylic acid compounds. The temperature ofthe contacting step preferably is kept low enough to minimize resindegradation. In one embodiment, the contacting is conducted at atemperature ranging from 25° C. to 120° C., e.g., from 25° C. to 100° C.or from 50° C. to 100° C. Some cationic macroreticular resins typicallybegin significant degrading (via the mechanism of acid-catalyzedaromatic desulfonation) at temperatures of 150° C. Carboxylic acidshaving up to 5 carbon atoms, e.g., up to 4 carbon atoms, or up to 3carbon atoms, remain liquid at these temperatures. Thus, the temperatureduring the contacting should be maintained below the degradationtemperature of the resin utilized. In some embodiments, the operatingtemperature is kept below temperature limit of the resin, consistentwith liquid phase operation and the desired kinetics for lithium and/orhalide removal.

The configuration of the resin beds within an acetic acid purificationtrain may vary, but the cationic exchanger should be upstream of themetal-exchanged resin. In a preferred embodiment, the resin beds areconfigured after a final drying column. Preferably the resin beds areconfigured in a position wherein the temperature of the crude acidproduct is low, e.g., less than 120° C. or less than 100° C. The streamcontacting the acid-form cationic exchange resin and the streamcontacting the metal-exchanged resin can be adjusted to the same ordifferent temperatures. For example, the stream contacting the acid-formcationic exchange resin can be adjusted to a temperature from 25° C. to120° C., e.g., 30° C. to 100° C., 25° C. to 85° C., 40° C. to 70° C.,e.g., 40° C. to 60° C., while the stream contacting the metal-exchangedresin can be adjusted to a temperature from 50° C. to 100° C., e.g.,from 50° C. to 85° C., from 55° C. to 75° C., or from 60° C. to 70° C.Aside from the advantages discussed above, lower temperature operationprovides for less corrosion as compared to higher temperature operation.Lower temperature operation provides for less formation of corrosionmetal contaminants, which, as discussed above, may decrease overallresin life. Also, because lower operating temperatures result in lesscorrosion, vessels advantageously need not be made from expensivecorrosion-resistant metals, and lower grade metals, e.g., standardstainless steel, may be used.

Referring back to FIG. 1, drying column bottoms stream 146 is firstpassed through cationic exchange resin bed 180 to remove lithium ions.Although one cationic exchange resin bed 180 is shown, it should beunderstood that a plurality of cationic exchange resin beds may be usedin series or parallel. The cationic exchangers may also remove othercations present in the stream, such as potassium, if added via line 160to drying column 130 as a potassium salt selected from the groupconsisting of potassium acetate, potassium carbonate, and potassiumhydroxide, and corrosion metals. Using the cationic exchangers of thepresent invention, the intermediate acetic acid product comprises lesslithium than the crude acetic acid product. In embodiments, theintermediate acetic acid product contains less than or equal to 5 wt. %of lithium present in crude acetic acid product, e.g., less than orequal to 3 wt. %, less than or equal to 2%, less than or equal to 1.5%,or less than or equal to 1%, less than or equal to 0.5%. Thus, when thecrude acetic acid product comprises lithium in an exemplary amount of400 wppb, the intermediate acid product may comprise less than or equalto 20 wppb lithium. In embodiments, intermediate acid product 182comprises a reduced amount of lithium in an amount of less than lessthan or equal to 100 wppb, e.g., less than or equal to 50 wppb, lessthan or equal to 20 wppb, or less than or equal to 10 wppb. Providing areduced amount of lithium advantageously maintains the efficient and/orcapacity of the metal-exchanged ion exchange resin bed 184 to removeiodides.

The resulting exchanged stream, e.g., intermediate acid product 182,then passes through a metal-exchanged ion exchange resin bed 184 havingacid cation exchange sites comprising at least one metal selected fromthe group consisting of silver, mercury, palladium and rhodium to removeiodides from the stream to produce a purified product 186. Although onemetal-exchanged ion exchange resin bed 184 is shown, it should beunderstood that a plurality of metal-exchanged ion exchange resin bedsmay be used in series or parallel. In addition to the resin beds, heatexchangers (not shown) may be located before either resin bed to adjustthe temperature of the stream 146 and 182 to the appropriate temperaturebefore contacting the resin beds. Similarly in FIG. 2, the crude aceticacid product is fed to cationic exchange resin bed 180 from side stream170. Heat exchangers or condensers may be located before either resinbed to adjust the temperature of the stream 170 to the appropriatetemperature before contacting the resin beds.

In one embodiment, the flow rate through the resin beds ranges from 0.1bed volumes per hour (“BV/hr”) to 50 BV/hr, e.g., 1 BV/hr to 20 BV/hr orfrom 6 BV/hr to 10 BV/hr. A bed volume of organic medium is a volume ofthe medium equal to the volume occupied by the resin bed. A flow rate of1 BV/hr means that a quantity of organic liquid equal to the volumeoccupied by the resin bed passes through the resin bed in a one hourtime period.

A purified acetic acid composition is obtained as a result of the resinbed treatment. The purified acetic acid composition, in one embodiment,comprises iodides in an amount of less than or equal to 100 wppb, e.g.,less than or equal to 90 wppb, less than or equal to 50 wppb, less thanor equal to 25 wppb, or less than or equal to 15 wppb. In oneembodiment, the purified acetic acid composition comprises lithium in anamount of less than or equal to 100 wppb, e.g., less than or equal to 50wppb, less than or equal to 20 wppb, or less than or equal to 10 wppb.In terms of ranges, the purified acetic acid composition may comprisefrom 0 to 100 wppb iodides, e.g., from 0 to 50 wppb, from 1 to 50 wppb,from 2 to 40 wppb; and/or from 0 to 100 wppb lithium, e.g., from 1 to 50wppb, from 2 to 40 wppb. In other embodiments, the resin beds remove atleast 25 wt. % of the iodides from the crude acetic acid product, e.g.,at least 50 wt. % or at least 75 wt. %. In one embodiment, the resinbeds remove at least 25 wt. % of the lithium from the crude acetic acidproduct, e.g., at least 50 wt. % or at least 75 wt. %.

One advantage of the present is to reduce a metal displaced from themetal-exchanged ion exchange resin, e.g. silver, mercury, palladium andrhodium, that undesirably accumulate in the purified acetic acid as thefinal product when no cationic exchanger is used to removed lithiumderived from and/or generated by the lithium compound in the reactionmedium. In one embodiment, the purified acetic acid comprises a metaldisplaced from the metal-exchanged ion exchange resin, e.g., silver,mercury, palladium and rhodium, in an amount of less than or equal to100 wppb, e.g., less than or equal to 90 wppb, less than or equal to 80wppb, less than or equal to 70 wppb, less than or equal to 60 wppb, lessthan or equal to 50 wppb, less than or equal to 40 wppb, less than orequal to 30 wppb, or less than or equal to 20 wppb. In terms of ranges,the purified acetic acid comprises a metal displaced from themetal-exchanged ion exchange resin, e.g., silver, mercury, palladium andrhodium, in an amount from 0 to 100 wppb, e.g., from 0.1 to 100 wppb,from 0.5 to 90 wppb, from 1 to 80 wppb, from 1 to 70 wppb, from 1 to 60wppb, from 1 to 50 wppb, from 1 to 40 wppb, from 1 to 30 wppb, or from 1to 20 wppb.

In one embodiment, there is provided a process for producing acetic acidcomprising carbonylating at least one member selected from the groupconsisting of methanol, dimethyl ether, and methyl acetate in thepresence of water in an amount from 0.1 to 14 wt. %, a rhodium catalyst,methyl iodide and a lithium iodide, to form a reaction medium in areactor, separating the reaction medium to form a liquid recycle streamand a vapor product stream, separating the vapor product stream in up to2 distillation columns in a primary purification train to produce acrude acid product comprising acetic acid and lithium derived fromand/or generated by the lithium compound in the reaction medium whereinthe lithium is in an amount of less than or equal to 100 wppm,contacting the crude acetic acid product with a cationic exchanger inthe acid form to produce an intermediate acid product comprising lithiumin an amount of less than or equal to 100 wppb, provided that the amountof lithium in the intermediate acid product is less than the crudeacetic acid product, and contacting the intermediate acetic acid productwith a metal-exchanged ion exchange resin having acid cation exchangesites to produce a purified acetic acid comprising a metal displacedfrom the metal-exchanged ion exchange resin in an amount of less than orequal to 100 wppb, lithium in an amount of less than or equal to 100wppb, and iodide in an amount of less than or equal to less than orequal to 100 wppb.

Distillation

The distillation columns of the present invention may be a conventionaldistillation column, e.g., a plate column, a packed column, and others.Plate columns may include a perforated plate column, bubble-cap column,Kittel tray column, uniflux tray, or a ripple tray column. For a platecolumn, the theoretical number of plates is not particularly limited anddepending on the species of the component to be separate, may include upto 80 plates, e.g., from 2 to 80, from 5 to 60, from 5 to 50, or morepreferably from 7 to 35. The distillation column may include acombination of different distillation apparatuses. For example, acombination of bubble-cap column and perforated plate column may be usedas well as a combination of perforated plate column and a packed column.

The distillation temperature and pressure in the distillation system cansuitably be selected depending on the condition such as the species ofthe objective carboxylic acid and the species of the distillationcolumn, or the removal target selected from the lower boiling pointimpurity and the higher boiling point impurity according to thecomposition of the feed stream. For example, in a case where thepurification of acetic acid is carried out by the distillation column,the inner pressure of the distillation column (usually, the pressure ofthe column top) may be from 0.01 to 1 MPa, e.g., from 0.02 to 0.7 MPa,and more preferably from 0.05 to 0.5 MPa in terms of gauge pressure.Moreover, the distillation temperature for the distillation column,namely the inner temperature of the column at the temperature of thecolumn top, can be controlled by adjusting the inner pressure of thecolumn, and, for example, may be from 20 to 200° C., e.g., from 50 to180° C., and more preferably from 100 to 160° C.

The material of each member or unit associated with the distillationsystem, including the columns, valves, condensers, receivers, pumps,reboilers, and internals, and various lines, each communicating to thedistillation system may be made of suitable materials such as glass,metal, ceramic, or combinations thereof, and is not particularly limitedto a specific one. According to the present invention, the material ofthe foregoing distillation system and various lines are a transitionmetal or a transition-metal-based alloy such as iron alloy, e.g., astainless steel, nickel or nickel alloy, zirconium or zirconium alloythereof, titanium or titanium alloy thereof, or aluminum alloy. Suitableiron-based alloys include those containing iron as a main component,e.g., a stainless steel that also comprises chromium, nickel, molybdenumand others. Suitable nickel-based alloys include those alloys containingnickel as a main component and one or more of chromium, iron, cobalt,molybdenum, tungsten, manganese, and others, e.g., HASTELLOY™ andINCONEL™. Corrosion-resistant metals may be particularly suitable asmaterials for the distillation system and various lines.

As is evident from the figures and text presented above, a variety ofembodiments are contemplated.

E1. A process for producing acetic acid comprising:

-   -   carbonylating at least one member selected from the group        consisting of methanol, dimethyl ether, and methyl acetate in        the presence of water in an amount from 0.1 to 14 wt. %, a        rhodium catalyst, methyl iodide, and a lithium compound, to form        a reaction medium in a reactor;    -   separating the reaction medium to form a liquid recycle stream        and a vapor product stream;    -   separating the vapor product stream in up to 2 distillation        columns in a primary purification train to produce a crude acid        product comprising acetic acid and lithium;    -   contacting the crude acetic acid product with a cationic        exchanger in the acid form to produce an intermediate acid        product; and    -   contacting the intermediate acetic acid product with a        metal-exchanged ion exchange resin having acid cation exchange        sites to produce a purified acetic acid.        E2. The process of embodiment E1, wherein the lithium in the        crude acid product is derived from and/or generated by the        lithium compound in the reaction medium.        E3. The process of any one of embodiments E1 or E2, wherein the        crude acid product comprises lithium in an amount of less than        or equal to 10 wppm.        E4. The process of any one of embodiments E1-E3, wherein the        crude acid product comprises water in an amount of less than or        equal to 0.2 wt. %.        E5. The process of any one of embodiments E1-E4, wherein the        intermediate acetic acid product comprises less lithium than the        crude acetic acid product.        E6. The process of any one of embodiments E1-E5, wherein the        intermediate acid product comprises lithium in an amount of less        than or equal to 100 wppb.        E7. The process of any one of embodiments E1-E6, wherein the        purified acetic acid comprises lithium in an amount of less than        or equal to 100 wppb.        E8. The process of any one of embodiments E1-E7, wherein the        purified acetic acid comprises a metal displaced from the        metal-exchanged ion exchange resin in an amount of less than or        equal to 100 wppb.        E9. The process of any one of embodiments E1-E8, wherein the        purified acetic acid comprises iodides in an amount of less than        or equal to 100 wppb.        E10. The process of any one of embodiments E1-E9, wherein the        vapor product stream is separated in 2 distillation columns in        the primary purification train.        E11. The process of any one of embodiments E1-E10, wherein the        cationic exchanger in the acid form comprises a resin of        acid-form strong acid cation exchange macroreticular,        macroporous or mesoporous resins.        E12. The process of any one of embodiments E1-E11, further        comprising a step of adding a potassium salt selected from the        group consisting of potassium acetate, potassium carbonate, and        potassium hydroxide to the distilled acetic acid product prior        to distilling the distilled acetic acid product in a second        distillation column; wherein at least a portion of the potassium        is removed by the cationic exchanger in the acid form.        E13. The process of any one of embodiments E1-E12, further        comprising adjusting the temperature of the crude acetic acid        product to from 50° C. to 120° C.        E14. The process of any one of embodiments E1-E12, further        comprising adjusting the temperature of the intermediate acetic        acid product to from 50° C. to 85° C.        E15. The process of any one of embodiments E1-E14, wherein        separating the vapor product stream comprises distilling the        vapor product stream in a first distillation column to form a        sidedraw comprising acetic acid; and distilling the sidedraw in        a second distillation column to produce a crude acid product        comprising acetic acid and lithium.        E16. The process of embodiment E15, wherein the crude acid        product is removed from a side stream port at a position above        the bottom of the second distillation column.        E17. The process of embodiment E16, wherein the side stream is a        liquid stream.        E18. The process of embodiment E15, wherein the crude acid        product is removed as a residue from the bottom of the second        distillation column.        E19. The process of embodiment E15, further comprising        condensing an low boiling point overhead obtained from the first        distillation column to form a heavy liquid phase and a light        liquid phase, and wherein a portion of the heavy liquid phase is        treated to remove at least one permanganate reducing compound        selected from the group consisting of acetaldehyde, acetone,        methyl ethyl ketone, butylaldehyde, crotonaldehyde, 2-ethyl        crotonaldehyde, 2-ethyl butyraldehyde, and the aldol        condensation products thereof.        E20. A process for producing acetic acid comprising:    -   carbonylating at least one member selected from the group        consisting of methanol, dimethyl ether, and methyl acetate in        the presence of water in an amount from 0.1 to 14 wt. %, a        rhodium catalyst, methyl iodide and iodide salts, to form a        reaction medium in a reactor;    -   separating the reaction medium to form a liquid recycle stream        and a vapor product stream;    -   separating the vapor product stream in up to 2 distillation        columns in a primary purification train to produce a crude acid        product comprising acetic acid comprising at least one cation        selected from the group consisting of Groups IA and IIA of the        periodic table, quaternary nitrogen cations, and        phosphorous-containing cations, wherein the at least one cation        is derived from and/or generated by a compound in the reaction        medium;    -   contacting the crude acetic acid product with a cationic        exchanger in the acid form to produce an intermediate acid        product; and    -   contacting the intermediate acetic acid product with a        metal-exchanged ion exchange resin having acid cation exchange        sites to produce a purified acetic acid.        E21. A process for removing iodides from a liquid composition        comprising:    -   a carboxylic acid or an anhydride thereof, greater than 10 wppb        of C₁₀-C₁₄ alkyl iodides, iodide anions, and a cation selected        from the group consisting of Group IA and IIA metals and        quaternary nitrogen cations, and quaternary        phosphorous-containing cations,    -   contacting said liquid composition with a cationic exchanger in        the acid form to produce an intermediate product with a reduced        concentration of cations selected from the group consisting of        Group IA and IIA metals, quaternary nitrogen cations, and        phosphorous-containing cations; and    -   contacting the intermediate product with a metal-exchanged ion        exchange resin having acid cation exchange sites comprising at        least one metal selected from the group consisting of silver,        mercury, palladium and rhodium to produce a purified acetic acid        product.        E22. The process of any one of embodiments E21-E22, wherein the        cation comprises lithium.        E23. The process of any one of embodiments E1-E22, wherein        metal-exchanged ion exchange resin comprises at least one metal        selected from the group consisting of silver, mercury, palladium        and rhodium.        E24. The process of any one of claims E1-E23, wherein at least        1% of the strong acid exchange sites of said metal-exchanged        resin are occupied by silver.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

EXAMPLES

The present invention will be better understood in view of the followingnon-limiting examples. A crude acetic acid product was obtained afterdistilling a vapor product stream in two distillation columns in aprimary purification train. The two distillation columns included alight ends column and drying column. The crude acetic acid product wasessentially acetic acid and had less than 0.15 wt. % water. The crudeacetic acid product was measured for Li and Ag content are in wppb, asshown in the following table. Comparative A-C were run without cationicexchanger in the acid form, which inventive Examples 1-3 were run with aAmberlyst-15 cationic resin. The outlet is the purified product from themetal-ion exchanged resin, namely a Ag-functionalized Amberlyst-15cationic resin. The high amounts of lithium in the outlet in ComparativeA-C correlate to an increase Ag concentration. Once the crude product ispassed through the cationic exchanger in the acid form, the lithiumconcentration decrease significantly, and this results in lower Agconcentrations in the outlet. In Table 4, ND stands for not detectable.

TABLE 4 Outlet from Metal- Crude Exchanged Acetic Cationic Ion AcidExchanger Exchange Product Outlet Resin Li Ag Li Li Ag Example (wppb)(wppb) (wppb) (wppb) (wppb) A 424 ND — 131 1207 B 346 ND — 267 1415 C454 ND — 353 1241 1 208 ND ND ND 30 2 422 ND ND ND 25 3 248 ND ND ND 20

We claim:
 1. A process for producing acetic acid comprising:carbonylating at least one member selected from the group consisting ofmethanol, dimethyl ether, and methyl acetate in the presence of 0.1 toless than 14 wt. % water, a rhodium catalyst, methyl iodide and iodidesalts, lithium acetate maintained at greater than or equal to 0.3 wt. %,to form a reaction medium in a reactor; separating the reaction mediuminto a liquid recycle stream and a vapor product stream comprisinglithium cations; and separating the vapor product stream in a primarypurification train consisting of a first column and second column,wherein the vapor product stream introduced to a first column to obtaina low-boiling overhead stream and side stream comprising acetic acid andlithium cations, and the side stream is introduced to a second column toobtain a bottoms stream comprising acetic acid, water, C₁₀-C₁₄ alkyliodides in an amount of greater than or equal to 10 wppb, and lithium inan amount from 0.01 wppm to 10 wppm, wherein the lithium in the bottomsstream is derived from and/or generated by a lithium compound in thereaction medium.
 2. The process of claim 1, wherein the bottoms streamcomprises lithium in an amount from 0.05 wppm to 5 wppm.
 3. The processof claim 1, wherein the lithium acetate is maintained in an amount from0.3 to 0.7 wt. %.
 4. The process of claim 1, wherein the side streamfurther comprises water, methyl acetate, and one or more C₁-C₁₄ alkyliodides.
 5. The process of claim 4, wherein the concentration of the oneor more C₁-C₁₄ alkyl iodides and the concentration of methyl acetate iswithin 0.9% of the water concentration in the side stream.
 6. Theprocess of claim 1, further comprising reducing the lithium amount inthe bottoms stream to less than 100 wppb.
 7. The process of claim 1,further comprising reducing the lithium amount by greater than 90 wt. %.8. The process of claim 1, wherein the bottoms stream comprises lessthan 0.2 wt. % water.
 9. The process of claim 1, wherein the bottomsstream further comprises hydrogen iodide.
 10. The process of claim 1,further comprising separating the low-boiling overhead stream into alight phase and heavy phase.
 11. The process of claim 10, furthercomprising removing one or more PRC from light phase and/or heavy phasein a distillation column outside of the primary purification train.